forbidden fruit

Unlawful pleasure or enjoyment; illicit love. For example, After Mary moved in with John, Tom began courting her—forbidden fruit is sweet, I guess, or Smoking behind the woodshed, that's a case of forbidden fruit. This expression alludes to Adam and Eve's violation of God's commandment not to touch fruit from the tree of knowledge, which resulted in their expulsion from the Garden of Eden (Genesis 3:6). In the form
forbidden fruit is sweet it appeared in numerous early English proverb collections.